It is known that waterproofing sheets suitable, e.g., for accomplishing roofings for buildings and industrial coverings, as well as coverings for water basins and landfills, are obtained by means of the impregnation of felts, or of fabrics, or of non-woven fabrics made from fibres of either natural or synthetic materials, with bitumen in the molten state.
This latter can be constituted by bitumen of distilled type, modified by means of the addition of certain amounts of amorphous plastomeric olefinic hompolymers and copolymers, such as polypropylene, or of elastomeric, thermoplastic ethylene/propylene copolymers, or styrene/butadiene/styrene copolymers.
The addition of such materials makes it possible bitumens to be obtained, which are endowed with high mechanical and elastic properties. However, with the supports used heretofore (generally of fiberglass or of fibres of polyester resins), which typically have extremely low elastic properties, the elastic characteristics of the bituminous component remain unused, so that end structures are obtained, the characteristics of which are practically anelastic. Furthermore, the supports of the above said type show a poor resistance to ageing and do not display any barrier properties for water, once that the layer of bitumen loses its continuity owing to stresses, yieldings of the structures and of the waterproofing materials, or owing to an incorrect application.